Apart from proving your intent to remain in Canada, if you go back to the US, the authorities will send approval's to your Canada address. Not sure how will you manage to get the correspondence if you leave Canada.
From what I know, if your wife is visiting you in US and then again getting a tourist/temporary visa to Canada to be with you, you can apply for her PR after you re-locate to Canada Permanantly. But remember, if you are a PR the immigration officers might(chances are there) reject your wife's Tourist visa because her intentions to remain in Canada are permanant. Specially from South asian countries, I have not seen many (of course there are exceptions) tourist visa approvals, while the PR process is about to start/in progress. So, check this a little more.
Also, read the below link, I think it will answer your doubts.
http://www.cic.gc.ca/english/information/applications/guides/5289ETOC.aspI hope someone with more knowledge would be able to suggest a better solution.